Competition between weak OH⋯π and CH⋯O hydrogen bonds: THz spectroscopy of the C₂H₂-H₂O and C₂H₄-H₂O complexes

THz absorption spectra have been recorded for the weakly bound molecular complexes of H₂O with C₂H₄ and C₂H₂ embedded in cryogenic neon matrices at 2.8 K. The observation and assignment of a large-amplitude acceptor OH librational mode of the C₂H₂-H₂O complex at 145.5 cm^-1 confirms an intermolecular CH⋯O hydrogen-bonded configuration of C_2v symmetry with the H₂O subunit acting as the hydrogen bond acceptor. The observation and assignment of two large-amplitude donor OH librational modes of the C₂H₄-H₂O complex at 255.0 and 187.5 cm^-1, respectively, confirms an intermolecular OH⋯π hydrogen-bonded configuration with the H₂O subunit acting as the hydrogen bond donor to the π-cloud of C₂H₄. A (semi)-empirical value for the change of vibrational zero-point energy of 4.0-4.1 kJ mol^-1 is proposed and the combination with quantum chemical calculations at the CCSD(T)-F12b/aug-cc-pVQZ level provides a reliable estimate of 7.1 ± 0.3 kJ mol^-1 for the dissociation energy D₀ of the C₂H₄-H₂O complex. In addition, tentative assignments for the two strongly infrared active OH librational modes of the ternary C₂H₄-HOH-C₂H₄ complex having H₂O as a doubly OH⋯π hydrogen bond donor are proposed at 213.6 and 222.3 cm^-1. The present findings demonstrate that the relative stability of the weak hydrogen bond motifs is not entirely rooted in differences of electronic energy but also to a large extent by differences in the vibrational zero-point energy contributions arising from the class of large-amplitude intermolecular modes.